Galaxy group

Many galaxies live together in groups.

These groups are very common. They have about 50 or fewer members. Each member is very bright.

Our home is in a group too. It is called the Local Group.

Groups stay close because of pull. This pull keeps them together. Some groups are very small and tight.

Some groups are old. They happen when galaxies crash and join. These are called fossil groups. They are very interesting to study.

Many galaxies live together in groups.

These are the most common structures in the universe. At least half of all galaxies live in groups. A group usually has 50 or fewer members. Each galaxy is very bright. Our own Milky Way is in the Local Group.

Groups stay together because of gravity. This is a pull that holds things close. Some groups are very tight. We call these compact groups. They often have only five galaxies. These groups show us how dark matter works. Dark matter is a type of matter we cannot see. It provides the extra pull to hold the group together.

Some groups are very old. These are called fossil groups. They form when galaxies crash and merge. This joining is called dynamical friction. In a fossil group, the big galaxies have joined into one. They leave behind a glow of X-rays. One fossil group is named NGC 6482. It is in the constellation Hercules. It is 180 million light-years away.

Many galaxies live together in small collections called galaxy groups. These groups are the most common structures in the universe. In fact, at least 50% of galaxies live in groups. A group usually has 50 or fewer members. Each member is at least as bright as our Milky Way.

These groups are smaller than galaxy clusters. Groups can even join together to form superclusters. Our own Milky Way is part of a group called the Local Group.

Gravity is the force that holds these galaxies together. Most groups are about 1 to 2 megaparsecs wide. Their total mass is about 10 to the 13th power of solar masses. The galaxies inside move at speeds of about 150 km/s. Some groups show a glow of X-rays. This glow comes from the gas between the galaxies. This happens in about half of the groups in our local universe.

Some groups are very tightly packed. We call these compact groups. They often have only about five galaxies close together. Astronomers found the first one in 1877. It is called Stephan's Quintet. This group has four galaxies and one extra galaxy nearby. In 1982, Paul Hickson made a list of these groups.

These tight groups help us see dark matter. We know dark matter is there because of its gravity. The visible mass is not enough to hold them together.

Over a long time, galaxies in a group can crash into each other. This process is called dynamical friction. When galaxies merge, they can create a fossil group. These are very old and quiet systems. The large galaxies have all joined into one central galaxy. They leave behind a glow called an X-ray halo. One fossil group is named NGC 6482. It is in the constellation Hercules. It is about 180 million light-years away.

Scientists also study groups that are just starting to form. These are called proto-groups. They are smaller than protoclusters. These young groups contain galaxies and even smaller protogalaxies. They are surrounded by dark matter. This matter is slowly fusing into one large shape. Watching these groups helps us learn how the universe grows. It is like watching a neighborhood being built.

A galaxy group is a collection of galaxies held together by gravity. These groups are the most common structures in our universe. Scientists estimate that at least 50% of all galaxies live in these groups. Each member of a group is quite large and bright. Specifically, every galaxy in a group is at least as luminous as the Milky Way. The Milky Way is our home galaxy, and it belongs to a collection called the Local Group.

Galaxy groups are much smaller than galaxy clusters. While clusters are massive, groups are considered the smallest aggregates of galaxies. A typical group contains about 50 or fewer members. These groups usually span a diameter of 1 to 2 megaparsecs (Mpc). The total mass of a group is approximately 10 to the 13th power of solar masses. Within these groups, individual galaxies move at a velocity spread of about 150 km/s.

Many groups show special signs of energy in the space between galaxies. In the local universe, about half of all groups exhibit diffuse X-ray emissions. This radiation comes from the intracluster media, which is the gas located between galaxies. These X-ray emissions usually appear within the inner 10% to 50% of a group's virial radius. This area typically covers a distance of 50 to 500 kiloparsecs (kpc). Groups that show these X-rays often contain early-type galaxies as their members.

Some groups are much more crowded than others. These are known as compact groups. A compact group usually contains only about five galaxies. These galaxies stay in close proximity and are relatively isolated from other large formations. Astronomers first discovered a compact group in 1877, known as Stephan's Quintet. This specific group actually consists of four galaxies and one unassociated foreground galaxy. In 1982, an astronomer named Paul Hickson created a famous catalogue of these groups called the Hickson Compact Groups.

Compact groups are very useful for studying the invisible parts of our universe. They provide clear evidence of dark matter. Scientists know dark matter exists because the visible mass in these groups is too low. Without dark matter, there would not be enough gravity to hold the galaxies together. Furthermore, compact groups are not dynamically stable over a Hubble time. This means they change over the age of the universe. This instability shows that galaxies evolve by merging with one another.

When galaxies merge over a very long time, they can create a fossil group. This happens through a physical process called dynamical friction. As galaxies interact, they eventually merge into a single, large central galaxy. This leaves behind an X-ray halo from the original group. Fossil groups are considered old, undisturbed systems. They have not seen many new large galaxies arrive since their initial collapse. One example is the elliptical galaxy NGC 6482. It is located in the constellation Hercules, about 180 million light-years away.

There are also groups that are just beginning to exist. These are called proto-groups. They are the smaller versions of protoclusters. Proto-groups consist of galaxies and protogalaxies that are still forming. These early structures are embedded in dark matter haloes. Over time, these haloes fuse together to create singular, larger dark matter structures. Studying these helps scientists understand how the large-scale structures of the universe are built.